A stress-laminated bridge deck behaves like a solid plate with a width equal to the bridge width and a length equal to the span. The structural action of this deck differs from beam action in that stresses and strains are distributed in two rather than one direction (orthotropic behavior), which results in a strong and predictable structure. For long span bridges, this type of deck may be used to span between main girders or transverse floor beams.
This deck slab is formed from individual timbers placed side by side and then compressed tightly together with large lateral forces. High strength steel rods (thread bars), or tendons are usually used to provide these high forces in the neighborhood of 60,000 to 120,000 pounds per rod. Alternatively the tensioning members or rods may be made of high strength plastic, such as fiber glass reinforced plastic (fiber glass) or other plastics or polymers. These rods may be rigid, flexible or cable-like. Unlike bolt forces of the past used to hold laminated timber beams, frames, and trusses together where nuts on threaded bolts were wrench tightened, these high rod forces are produced with the use of hollow-core hydraulic jacks to very large precalculated design magnitudes, in a measured fashion. Such forces squeeze the timbers, greatly increasing frictional resistance between timbers and eliminating the need for mechanical connectors or glue used in various ways with laminated wood beams. As a consequence, increased strength properties and resistance to deflections are realized in the transverse as well as the longitudinal direction of the bridge.
Creep of the wood perpendicular to the grain occurs soon after jacking of the rods. Consequently, a second rod jacking is required after about 24 hours. Further creep has been found to occur very slowly. However, as a final safeguard, a third rod jacking is performed after about two months. Experience with existing bridges indicates that further jacking is unnecessary and that rod forces will be stable after the third jacking. Such strengthening allows timbers of short length to be butted at their ends in a staggered pattern to form the overall length of bridge deck.
The rod stressing and resulting transverse compression of the timbers improves bridge performance. This should not, however, be confused with the prestressing of timber beams and frames in flexure. No longitudinal flexural prestressing is imposed here prior to the application of bridge loads.
Stress-laminating was first used in Ontario, Canada in 1976. Since then this bridge type, without metal plates, has become popular in Canada and, more recently, in the United States. Results of tests conducted at the University of Wisconsin have shown that the major shortcoming of the stress-laminated bridge deck is lack of stiffness when used over a long span. The mode of failure is excessive deflection. Resulting timber stresses are usually well within allowable values. The Trout Road Bridge, built in May 1987 near Houserville, Pa., has been successfully monitored for one year. Dead and live load deflections, losses in bar forces, and moisture content of the creosoted timber deck were observed and analyzed. Results indicate a well-behaved and esthetically pleasing bridge type for short spans. However, measured live load deflections were found to be in excess of allowable deflections specified in highway bridge specifications. The 46' span of this bridge obviously required timbers to be butted together at intervals. The usual procedure has been to limit butt joints no closer than every fourth member at any given bridge cross-section. Large Douglas Fir timbers (4".times.16") with a maximum length of 20 feet were used. Such large dimensions are scarcely procurable in most sections of the country. To fully utilize smaller timber cross-sections and lengths, butt joints must be spliced such that resulting bridge deflections remain within allowable values.
Renewed interest in the use of wood for bridge construction has arisen because of its cost effectiveness compared with other materials. The U.S.D.A. Forest Service is particularly interested in stress-laminated structures because they can be constructed by in-house labor in a very short period of time. State and township governments are also interested in stressed timber bridges to economically replace thousands of deficient structures in a rapid and efficient manner. But before the stress-laminated bridge deck can be fully utilized, the lack-of stiffness (excessive deflection) shortcoming must be properly addressed.
The use of metal plates described in this invention offers the solution to the reduction of excessive deflections and provides other structural advantages as well.
An object of this invention is to produce a compound timber-metal stressed deck in which permanent set (creep) caused by long-time loads is minimized; camber is better retained and dead and live load deflections are reduced.
Another object of this invention is to produce a stressed deck in which longer simple spans are possible, and in which reduced depth of timbers is possible.
Yet another object of this invention is to produce a stressed deck having continuous spans with plates in regions of high moments, leading to economy of materials.
Yet another objects of this invention is to design a compound timber-metal stressed deck in which the transverse sag of the deck cross-section can be countered by the addition of extra metal plates where the sag is largest.
Still another object of this invention is to produce a compound stress deck in which orthotropic action is improved as well as flexural rigidities parallel and perpendicular to the direction of traffic with improved torsional rigidity.
Yet another object of this invention is to produce a bridge span wherein the transverse wheel load distribution is improved.
Still another object of this invention is to produce a bridge span utilizing shorter timber lengths wherein camber is easier to form.
Still another object of this invention is to produce a compound timber-metal bridge span with smaller and nearly square cross-sections employed in two or more layers, allowing smaller diameter trees to be utilized.
Another object of this invention is to produce a bridge span in which low grade timber may be effectively used in combination with metal plates and in which the loss in bridge stiffness at butt joints is minimized.
Yet another object of this invention is to produce a bridge deck in which stressed rod forces are more uniformly distributed transversely through the timbers when metal plates are employed, giving better friction distribution; also, a higher percentage of initial rod forces are retained which allows smaller rod forces with reduced damage to facia timbers caused by compressive pressure under the bearing plates.
Still another object of this invention is to design a timber deck bridge with improved vibrational characteristics wherein the metal plates cause the structure to have a higher natural frequency and a lower amplitude of vibration.
Yet another object of this invention is to produce simple bridge fabrication in which the metal fabrication consists of plate shearing and hole drilling only.
Yet another object of this invention is to produce a bridge span in which high strength steel plates can be shipped in convenient lengths and butt welded at the site in which no painting or galvanizing of the metal plates is required.
Another object of this invention is to build a bridge of reduced depth with less constriction to the effects of high water.
Yet another object of this invention is to construct a bridge of reduced depth which will allow for more economical design of abutments, piers, and approaches.
A final object of this invention is to produce a bridge span having natural beauty of the timber deck - the metal plates are hidden from view.